Orthogonal frequency division multiplexing (OFDM) is used to reduce an inter-symbol interference (ISI) caused by multi-fading in a wireless communication environment. OFDM is used to divide a high-speed data stream into multiple low-speed sub data streams and carry the sub data streams over multiple sub-carriers. Since symbol durations of low-speed sub-carriers increase in OFDM, relative signal dispersion due to delay dispersion of a multi-path decreases. Guide intervals longer than delay dispersion of a channel are inserted into all OFDM symbols to remove ISI. Therefore, each sub-carrier band in OFDM can be assumed to be a flat attenuation channel.
U.S. Pat. No. 6,163,766 discloses an adaptive modulation technique. In the adaptive modulation technique, the number of allocated transmission bits of information for each sub-carrier is adjusted according to the conditions of a channel. When the channel is in a high-quality state, a large number of transmission bits are allocated for each sub-carrier, and when the state of the channel is poor, a small number of transmission bits are allocated or no data is transmitted.
U.S. Pat. No. 5,940,439 discloses an adaptive modulation and coding (AMC) technique. Two requirements should be satisfied to realize AMC for carrying a large amount of data over limited frequency sources. First, the state of a channel should be accurately estimated. Next, estimated channel state information (CSI) should be signaled without time delay or errors.
OFMD that is robust against multi-path fading and adaptive modulation, which adjusts the amount of allocated transmission bits of information according to the conditions of a channel, are combined into adaptive OFDM. In the adaptive OFDM, CSI should be fed back over each sub-carrier, and the amount of information fed back via uplink increases. In addition, since the characteristics of a channel temporally continuously vary in wireless communication, CSI accounts for a high percentage of the total transmitted information, which results in a waste of wireless channel resources.
In Internet-based next generation wireless communications, asymmetric duplexing at a ratio of 4:1 is used in frequency bands in downlink and uplink in consideration an increase in a necessary data rate in a forward link. An adaptive OFMD system should use a considerable portion of uplink resources to feed back CSI. When an uplink channel is in a poor state, information fed back to a transmitter is damaged, resulting in deteriorating communication performance.
A multiple antenna may be used to improve adaptive OFDM and spectral efficiency. When using the multiple antenna for OFDM, information regarding a spatial multiplexing (SM) scheme and a transmission scheme such as a transmit diversity (TD) scheme or the like should be transmitted along with information regarding a modulation and coding scheme (MCS) using a single antenna. Here, the SM scheme is to transmit different types of data to multiple transmitter antennas, respectively, in the high-quality states of channels to maximize a data transfer rate. In the TD scheme, the same data is coded to be suitable for a delay diversity concept, replicated as a plurality of data, and transmitted via multiple antennas.
In order to efficiently use the multiple antennas, signaling information should include information on a transmission scheme used for a transmitter antenna. In an MCS using a single antenna, three bits are generally allocated for MCS level information, while in an MCS using multiple antennas, one bit plus three bits is used to distinguish the SM scheme from the TD scheme.
Frequency selective fading may be serious due to a multi-path delay in an outdoor wireless communication environment. Thus, in the outdoor wireless communication environment, the number of sub-carriers should be increased so that each of the sub-carriers experiences uniform fading. Thus, the amount of CSI to be fed back to a transmitter increases with an increase in the number of sub carriers. Also, since a mobile station (MS) can change its position quickly, the characteristics of a channel vary frequently so that a period for feeding the CSI back is shortened. As a result, load on an uplink increases.
CSI is fed back via an uplink with a predetermined period of time. Thus, CSI may be unnecessarily fed back even when very short sub data frames requiring no feedback of CSI are transmitted via a downlink.
Wireless resources may be wasted in either a frequency division duplex (FDD) or a time division duplex (TDD) due to feedback CSI. More specifically, in the case of the TDD, a receiver should receive information regarding data modulation format information being transmitted from a transmitter as modulation level information (MLI). Thus, a large amount of additional information is necessary. Accordingly, a technique for reducing feedback CSI is required for the FDD and the TDD.